The word "sick" is used primarily by teenagers and wannabees to mean "cool" or "fresh".
Sometimes is is said in a drawn out form for emphasis such as "s-i-i-i-i-i-i-i-ick".
It can also be used in mock "coolness" when someone falls or attempts to do something amazing and sucks instead.
A sick whistle is the act of saying the work "sick" twice in a row very quickly in a male falsetto voice.
"Sick Sick"

A "riced-out" Honda Civic drove past a group of older males and was subjected to a chorus of mock "Sick Whistles."